The use of light-emitting diode (LED) strings instead of fluorescent bulbs for use in illumination is drastically increasing based on consumer demands for better light quality. In addition, typical LED light efficacy can be much better than conventional lighting systems, thus consuming significantly less power. In addition, amongst other advantages, LED systems can be smaller and more environmentally friendly, and can have a faster response with less electro-magnetic interference (EMI) emissions. A number of LED regulation techniques exist for typical LED systems, such as constant-current regulation, constant-voltage regulation, and a combination of constant-current/constant-voltage regulation.
Many LED drivers (power supplies) provide a regulated constant current output which drives an LED string to create a constant light output. Most LED drivers contain a method to dim the light output from full scale to something less than full scale. The dimming can be done in a linear fashion by decreasing the regulated output current, or the dimming can be done in a pulse-width-modulation fashion by duty cycle modulation of the full scale regulated current. Photometrically, there are advantages to PWM dimming, since the color shift of the light output is reduced compared to linear dimming.
Early LED drivers were created by using voltage-regulator drivers adapted for current regulation. The PWM dimming was accomplished by modulating the driver's Enable pin, which cycled the entire driver on and off at a low frequency, and variable duty cycle. The PWM dimming performance of this approach suffered since the drivers internal circuitry (bandgap, internal voltage regulators, oscillator, etc.) was often times disabled as the driver was disabled. Therefore, when the driver was re-enabled there was an undesirable delay due to the driver initialization before it could provide a regulated output current. Other early LED drivers reused voltage-regulator driver features such as soft-start for PWM Dimming. The soft-start capacitor was actively discharged during PWM Dimming off-times, and released during PWM Dimming on-times. Soft-start also proved to have an undesirable delay because it limited the system's recovery response time during PWM Dimming.
In an attempt to maximize PWM Dimming performance, most LED drivers abandoned the conventional fixed frequency control method and instead moved to a hysteretic controller. In a hysteretic controller, the driver's output current is compared to two thresholds: a low threshold, and a high threshold. This improves PWM dimming performance quite drastically, and removed the error amplifier from the driver. However, with a hysteretic control method, a major drawback is that multiple LED drivers cannot be synchronized to a common oscillator. Other PWM Dimming approaches connect/disconnect the entire LED string via a power FET located in series with the LED. The LED is quickly opened by the FET, meaning its current goes to zero instantaneously. However, reconnecting the LED string can often cause overshoots in the LED current if the system is not well controlled. Another approach connects/disconnects the charged output capacitor via a power FET in series with the output capacitor. This preserves the output voltage during PWM dimming off-times, at the expense of a costly power FET.